Moisture behaviour and biological durability of wood‐polymer composites

In recent years wood-polymer composites appears at the European market more frequently, mainly in decking applications as an alternative for durable tropical hardwood decking. This study focuses on extruded decking products based on polypropylene (PP), polyethylene (PE) or polyvinylchloride (PVC) and wood flour. All products are commercially available on the Belgian market. Initially WPC’s were said to be resistant for biological degradation as the wood particles should be encapsulated by the polymer, but over the years several cases of fungal decay of WPC have been reported. Therefore the moisture behaviour of the different WPC decking materials was first assessed by various moistening methods to determine if the moisture content can reach levels that initiate fungal growth. Concerning this, WPC shows better results than the traditional wood composites like particle board, MDF or OSB, but clearly absorbs sufficient moisture to be critical if sorption time is long enough. Therefore, when biological durability is tested, an adequate standard, more specific for WPC products, is needed in stead of just copying the standards designed for wooden panel products. For WPC materials a moistening pre-treatment is needed prior to the proper fungal test to include moistening time as a critical factor for assessing biological durability of these materials. Furthermore placing the products in a fungal control unit that evaluates the susceptibility for airborne surface stains revealed different results and showed that fungi grew earlier and faster on weathered samples. Concluding, in spite of the different composition of the tested products, no product was significantly better or worse than the other concerning the moisture behaviour. Furthermore only a few products were to a small extent covered by moulds in the fungal control unit

Biologische duurzaamheid van hout-polymeer-composieten

De laatste decennia is een nieuw product gebaseerd op zowel hout als plastiek op de markt verschenen. Daar waar hout onder invloed van vocht niet alleen zwelt en krimpt maar ook nog gevoelig is voor houtrot is plastiek minder stabiel onder invloed van wisselende omgevingstemperaturen. De extrusieproducten op basis van polyethyleen (HDPE), polypropyleen (PP) of polyvinylchloride (PVC) en houtmeel vinden vooral hun toepassing in buitenterrasvloeren als alternatief voor duurzame tropische houtsoorten. Op deze manier komt hout uit onze bossen ook in aanmerking voor deze toepassing. Toch blijft de vraag of enkel door beide componenten samen te brengen alle risico op biodegradatie wordt uitgesloten. In dit onderzoek werden een reeks producten die aanwezig zijn op onze markt onderworpen aan verschillende regimes om aldus te achterhalen in welke mate het houtvocht beneden de gevaarzone voor biologische aantasting blijft. Deze ‘wood-plastic composites’ scoren in dit verband aanzienlijk beter dan traditionele houtcomposieten zoals spaanplaat, MDF of OSB. Het gebruik van composieten op basis van een kunststofmatrix en een ligno-cellulose vulling of vezelmateriaal wordt in een algemene context steeds in vraag gesteld wanneer de eindproducten in buitenomstandigheden of in vochtige omgeving worden gebruikt. Afsluiten of verhinderen van bevochtiging van hout of natuurlijke vezels in composieten is niet alleen een alternatief voor het gebruik van duurzame houtsoorten, maar laat ook toe hernieuwbare grondstoffen te gebruiken in toepassingen waar tot voor kort alleen ‘man-made materials’ werden voorzien

Non-destructive evaluation of wood decay

Evaluation of wood decay is often prone to subjective assessment. Standard rating scales are based on human perception of decay, often in addition to weight loss measurements. Especially the assessment of graveyard specimens or other long term testing material that has to be monitored regularly on a non-destructive basis, is challenging. In this paper two techniques are presented that can deliver extra insight. The first technique is based on the analysis of the resonance frequencies of the material. It is very fast and only minimal sample manipulation is required. Instantaneously, an approximation of the modulus of elasticity (MOE) and damping are obtained that can be followed during degradation. The technique is applicable in the field. The second method requires an X-ray tomography scanner, but enables a three dimensional view on the internal structure of the sample and accompanying degradation. It is not applicable in the field but can give a very interesting view on possible degradation patterns. Both methods can not only assist in non-destructive assessment, but can possibly best serve as early decay detection techniques and give more insight into the rate and modes of degradation

Moisture dynamics of WPC as basis for biological durability

The largest market for wood-polymer composites (WPCs) is currently decking. Although many products are commercially available, a proper standard for the assessment of the biological durability of WPC does not exist. Recommended standards for testing resistance against basidiomycetes should be completed with a method to bring the specimens in a worst case situation, obtaining a moisture level high enough to initiate and support fungal growth at the beginning of, or early in the test. In this study a simple, but efficient way to increase the moisture level of test specimens of 9 commercialized WPC decking products is presented. The 24 h air-drying period after 4 weeks immersion in warm water strongly reduced the high moisture content (MC) of the wood particles, but the induced thickness swell may allow a quicker re-moistening. Performing a fungal test with these moistened specimens, Coriolus versicolor caused 2 – 15% mass loss (ML) and Coniphora puteana 1 – 12% ML despite of the poor virulence of latter fungus. ML was correlated with the MC after incubation and the thickness swell due to the pre-treatment. The ability of fungal spores to settle on WPC surfaces was investigated as well by placing specimens in a cabinet loaded with mist saturated with fungal spores and an equivalent outdoor test setup. Linking results to the composition or production process is difficult and was not the main goal of this research, but PVC based materials clearly performed the worst: high water absorbing and thickness swell, the highest ML in lab-based fungal testing and the most distinct fungal staining during outdoor exposure